Method for treating surface of magnesium or magnesium alloy

ABSTRACT

A method for surface treatment is disclosed. The method is achieved by forming a MgO film on a metal surface through anode processing of Mg or Mg alloy in an alkaline solution. The alkaline solution includes a hydroxide, a thickening agent, and a film adjusting agent. As the method is performed, the target object is immersed in the alkaline solution, and the target object is connected to an anode with an average electric current density of 1˜5 A/dm, at a temperature of 0˜30° C., and within a time period of 10˜120 minutes to form a film of 5˜25 μm. The forming rate of the film of the method of the present invention is fast, and the formed film is of little stress.

FIELD OF THE INVENTION

The present invention relates to a surface treatment method,particularly a surface treatment method applicable on Mg or Mg Alloy.

BACKGROUND OF THE INVENTION

Mg alloys are high in strength and light in weight and are widely usedon airplanes, vehicles, and electronic products. Since magnesium iscapable of forming alloys of high strength with many types of metal, Mgalloys have a wide variety of applications. However, Mg alloys aregenerally not suitable for mass production due to their disadvantages,such as poor corrosion resistance, and poor wear resistance, etc.Furthermore, due to an ever increasing expansion on the applications ofMg alloys, the demand on the acid-corrosion-resistance of the alloys isalso increasing day by day.

In the past, a Mg alloy is usually protected against acid corrosion bycoating a protective paint or forming a protective film on the surfaceof the alloy. In recent years, due to the improvement in technology, theuse of forming a protective film on the surface of a Mg alloy has becomethe main stream technique.

Traditionally, a micro-arc oxidation treatment is used to form aprotective film on a Mg alloy. Such a technique is characterized in thata high voltage exceeding 600˜1000V is used, the treatment temperature isover 40° C., and a film forming electrolysis is carried out in afluoride-containing weak alkaline electrolysis solution. However, asurface formed through such a technique is rough due to the formation ofa large amount of penetration sparks, and requires an additional coatingtreatment. Furthermore, due to the use of fluoride as a main chemicalagent, wastewater from such a treatment is difficult to be treated andhas a greater pollution impact on the environment.

Moreover, acidic mixture solvents, e.g. borate, sulfate, phosphorateions, fluorate ions, and chlorine ions, etc., are used for acidic anodictreatment to form a protective film. However, since a Mg alloy dissolvesrapidly in an acidic state, the surface of the resulting film is liableto become rough. As a result, the dimensional precision of the workpieceis affected, a large residual internal stress is developed, and theprocess variables have a narrow window.

Therefore, at present, the industry is urgently in need of a Mg or Mgalloy surface treatment method to overcome the above-mentionedconventional drawbacks without the need of using high temperature andhigh pressure and capable of forming an oxidation film rapidly.

SUMMARY OF THE INVENTION

A main objective of the present invention is to provide a surfacetreatment method capable of forming a uniform anodic film on theMg-containing material, e.g. Mg or Mg alloy.

A surface treatment method according to the present invention comprisesthe following steps: providing a metal of Mg or Mg Alloy, a tank, and asurface treatment composition in the tank, wherein said surfacetreatment composition includes a hydroxide, a film thickening agent, afilm adjustment agent, wherein said tank is equipped with an electrode;sequentially immersing said Mg or Mg Alloy or the surface thereof insaid surface treatment composition; flowing an electric current throughsaid Mg or Mg Alloy as an anode and the electrode immersed in saidsurface treatment composition via said surface treatment composition;and terminating the electric current and removing the treated Mg or MgAlloy from said tank.

Furthermore, in a preferred surface treatment method for a metal of Mgor Mg Alloy according to the present invention, the temperature duringthe surface treatment reaction is not limited, and is preferably 0˜40°C. In one embodiment, the pH value of the surface treatment compositionof the present invention is not limited, preferably larger than 9, morepreferably larger than 10, and most preferably larger than 11.

Preferably, said Mg and Mg alloy used in the present invention is aMg-riched alloy or casting Mg alloy. In a treatment method according tothe present invention, the average current density of the electriccurrent used is not limited, preferably is 1˜10 A/dm², and morepreferably is 1˜5 A/dm². Furthermore, the treatment time for the Mg orMg Alloy in the surface treatment composition of the present inventionis not limited, preferably is 5˜240 minutes, and more preferably is10˜120 minutes.

The film thickening agent used in the present invention can be anyconventional film thickening agent, preferably aluminate, silicate,vanadate, molybdate, tungstate, or a combination thereof, with anarbitrarily applicable concentration in the surface treatmentcomposition. In a preferred embodiment of the present invention, theconcentration of the film thickening agent is 10˜150 g/L. The mainobjective of using the film thickening agent is for growing a film. Thefilm forming rate increases along with an increase in the concentrationof the film thickening agent. Under suitable conditions, the objectiveof the film thickening can be achieved without the need of using hightemperature and ultra-high voltage.

A film adjustment agent suitable for use in the present invention is notparticularly limited, and is preferably potasium dihydrogen phosphate,sodium dihydrogen phosphate, tripotassium phosphate, trisodiumphosphate, oxalic acid, succinic acid, fatty acid, malic acid, or acombination thereof, with an arbitrarily applicable concentration in thesurface treatment composition. In a preferred embodiment of the presentinvention, the concentration of the film adjustment agent is 10˜300 g/L.The film adjustment agent is capable of accelerating the forming rate ofthe film, promoting the formation of a uniform and fine film, as well asreducing the stress in the film.

Furthermore, the hydroxide used in the present invention can be anyconventional hydroxide, preferably sodium hydroxide, potassiumhydroxide, or a mixture thereof, with an arbitrarily applicableconcentration in the surface treatment composition. In a preferredembodiment of the present invention, the concentration of the hydroxideis 10˜100 g/L.

In a preferred embodiment of the present invention, said Mg or Mg Alloyis connected to the positive electrode of a rectifier while undergoingthe surface treatment. However, in embodiments according to the presentinvention, the rectifier used is not limited, preferably is a d.c.rectifier, and a pulse rectifier, and more preferably is a pulserectifier. The d.c. rectifier, for example, can be a common constantcurrent, constant voltage, or constant current density type rectifier; aconstant current, constant voltage, or constant current density recycletype d.c. rectifier; or a constant current, constant voltage, orconstant current density PR-type d.c. rectifier. The pulse typerectifiers can be pulse type rectifiers of different wave forms. Thevoltage used in the method of the present invention is only 100˜300V,and the operation is carried out at room temperature at a low energyconsumption.

The method of the present invention uses an anode oxidation film formedon the surface of Magnesium to achieve corrosion resistance. Afteroxygen atoms are developed near the anode in the electrolyte solution,the atoms in the substrate migrate to the surface of the anode and forman oxidation film (film forming) on the substrate. The formed film isslightly dissolved by the electrolyte solution (chemical dissolution).An oxidation film starts to develop when the film formation rate isgreater than the film dissolution rate, thereby forming an oxidationfilm of the substrate. This is called an anode treatment.

In the present invention the Mg or Mg alloy is capable of forming a filmmainly constituted of magnesium oxide ceramic ingredient in an alkalinesolution by anodic oxidation treatment, and the magnesium oxide ceramicingredient is slightly soluble in an alkaline solution by chemicaldissolution. Since the surface treatment composition according to thepresent invention contains no fluoride, such a surface treatmentcomposition will not cause severe environmental pollution. Furthermore,the present invention uses a film thickening agent and a film adjustmentagent to achieve an increase in the film forming rate, a reduction infilm dissolution rate, a uniform and fine film, a stable dimensionalprecision of the workpiece, and a reduction on the internal stress ofthe film.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a cross-sectional schematic view of a film formed on thesurface of a metal of Mg or Mg alloy according to the method of thepresent invention;

FIG. 2 shows a Bode diagram of an electrochemical a.c. impedance testperformed on a film formed on the surface of a metal of Mg or Mg alloyaccording to a preferred example of the present invention;

FIG. 3 shows a TEM photo of an anode film formed on the surface of ametal of Mg or Mg alloy according to a preferred example of the presentinvention;

FIG. 4 shows a TEM photo of a locally enlarged A layer in FIG. 3; and

FIG. 5 shows a TEM photo of a locally enlarged C layer in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention uses a film thickening agent and a film adjustmentagent at different concentrations in an anodic treatment on a materialof Mg alloy AZ31 (wherein said Mg alloy includes more than 90% of Mg, 3%of Al, and 1% of Zn). Even though the present invention uses the Mgalloy AZ31 as illustrated in the following examples, the composition ofa Mg or Mg Alloy applicable in the present invention is not limited toAZ31, but is limited by the scope defined by the claims of the presentinvention. In one embodiment, the present invention uses silicate as thefilm thickening agent. In another embodiment, the present invention usesvanadate as the film thickening agent. In both cases, a film is capableof being formed on the surface of the Mg alloy. Furthermore, a filmthickening agent according to the present invention is not limited tosilicate and vanadate.

In a preferred embodiment, a method according to the present inventioncomprises: firstly, providing a hydroxide, a film thickening agent, anda film adjustment agent; mixing the above-mentioned chemical agents toform a surface treatment composition; loading the prepared surfacetreatment composition into an electrolysis tank; next, mounting a Mg orMg Alloy on a workpiece, and then mounting the workpiece on an anodelocation in the electrolysis tank containing said surface treatmentcomposition; using a rectifier to apply an electric current on saidanode in order to perform a film forming reaction on the surface of theMg or Mg Alloy; after a specified reaction time, removing the workpiecetogether with the Mg or Mg Alloy from the electrolysis tank; and washingthe surface of the Mg or Mg Alloy with water to complete a surfacetreatment operation for the Mg or Mg Alloy.

In an embodiment according to the present invention, the rectifier canbe a d.c. rectifier or a pulse rectifier. In an embodiment, a d.c.rectifier is set to a current density of 1˜5 A/dm²; in anotherembodiment, a pulse rectifier is set to a current density of 1˜5 A/dm²,a frequency of 10˜2000 Hz, and a duty cycle of 0.1˜1.

FIG. 1 shows a cross-section of a schematic diagram of a film formed onthe surface of the Mg or Mg Alloy according to a preferred embodiment ofthe present invention. The film formed on the surface of the Mg or MgAlloy according to the present invention is examined by anelectrochemical AC current impedance spectrum and a TEM. The examinationresults indicate that the film has a three-layered structure, includingtwo barrier layers and a porous layer. As shown in FIG. 1, the topmostlayer of the film is a porous layer containing MgO and Mg₂SiO₄, theintermediate layer is a barrier layer formed of a dense MgO structure,and the bottom layer is a barrier layer formed of a nano crystallineMgO, wherein the porous layer is advantageous for the anchoring of acoating for the corrosion resistance of the substrate, and the barrierlayers are capable of adjusting the film strength, and increasing thetoughness and corrosion resistance of the film. Therefore, a film formedaccording to the method of the present invention, due to themulti-layered structure thereof, has the functions of buffering theinternal stress of the film, accelerating the film forming rate, andincreasing the denseness and corrosion resistance of the film.

FIG. 2 shows a Bode diagram of an electrochemical a.c. impedanceexamination on a film formed on a Mg Alloy according to a preferredexample of the present invention. FIG. 2 indicates that peaks appear atlocations of 10⁻¹ Hz and 10² Hz frequency. This indicates that the filmformed by the anodic treatment according to the present invention hasseparately two layer structures at said locations. The frequency rangeof 10⁻³ Hz to 10⁴ Hz is the range for the barrier layers, and obviouslythere are only two peaks exist within this range, according to FIG. 2.This indicates that indeed the film has two barrier layers each of adifferent structure.

FIG. 3 is a TEM photo diagram of the film formed on a Mg alloy by theanodic treatment according to a preferred example of the presentinvention. From the diagram, the film formed according to the method ofthe present invention includes three different structures, the A, B, Clayers as shown in FIG. 3. A local enlargement diagram of the A layer isshown in FIG. 4, from which it can be seen that the A layer is a poroustopmost layer structure. The bright spots shown in FIG. 5, a localenlargement diagram of the C layer, are nano MgO crystals. Thus, the Clayer is a bottom layer structure with nano MgO crystals.

The reaction conditions and parameters of embodiments according to thepresent invention are shown in the following. The A-series examples usea d.c. rectifier for supplying electric current, and the B-seriesexamples use a pulse rectifier for supplying electric current.Furthermore, comparative Examples A, and B are control sets for theA-series examples, which use a d.c. rectifier; however and a surfacetreatment composition free of a film thickening agent or a filmadjustment agent. The anode film formed on the Mg-containing alloy inExample A1 has the best corrosion resistance among the films formed inthe A-series examples.

EXAMPLE A1

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and oxalic acid (80 g/L) wereused as a film adjustment agent. This example used a d.c. rectifier andadopted the following conditions: temperature 20° C., electric currentdensity 1.6 A/dm², and reaction time 30 minutes.

EXAMPLE A2

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium vanadate (50 g/L) was used as a film thickening agent,and trisodium phosphate (19 g/L) and oxalic acid (80 g/L) were used as afilm adjustment agent. This example used a d.c. rectifier and adoptedthe following conditions: temperature 20° C., electric current density1.6 A/dm², and reaction time 30 minutes.

Since sodium metasilicate is cheap and readily available, and theresulting anodic film has a fair performance, sodium metasilicate wasused as a film thickening agent in the following examples. Meanwhile,various film adjustment agents with different concentrations of agentswere used in the examples for investigating the role of each chemicalagent in the resulting anodic treated films.

EXAMPLE A3

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (21 g/L) was used as a film thickeningagent, and trisodium phosphate (95 g/L) and succinic acid (80 g/L) wereused as a film adjustment agent. This example used a d.c. rectifier andadopted the following conditions: temperature 20° C., electric currentdensity 1.6 A/dm², and reaction time 30 minutes.

EXAMPLE A4

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (95 g/L) and fatty acid (80 g/L) wereused as a film adjustment agent. This example used a d.c. rectifier andadopted the following conditions: temperature 20° C., electric currentdensity 1.6 A/dm², and reaction time 30 minutes.

EXAMPLE A5

Surface treatment composition: sodium hydroxide (10 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and malic acid (80 g/L) wereused as a film adjustment agent. This example used a d.c. rectifier andadopted the following conditions: temperature 20° C., electric currentdensity 1.6 A/dm², and reaction time 30 minutes.

EXAMPLE A6

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (57 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a d.c. rectifierand adopted the following conditions: temperature 20° C., electriccurrent density 1.6 A/dm², and reaction time 30 minutes.

EXAMPLE A7

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (48 g/L)were used as a film adjustment agent. This example used a d.c. rectifierand adopted the following conditions: temperature 20° C., electriccurrent density 1.6 A/dm², and reaction time 30 minutes.

EXAMPLE B1

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 1.6 A/dm², frequency 1000 Hz, duty cycle 0.3,and reaction time 15 minutes.

EXAMPLE B2

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 1.6 A/dm², frequency 1000 Hz, duty cycle 0.3,and reaction time 45 minutes.

EXAMPLE B3

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 45° C.,electric current density 1.6 A/dm², frequency 1000 Hz, duty cycle 0.3,and reaction time 15 minutes.

EXAMPLE B4

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 2.2 A/dm², frequency 1000 Hz, duty cycle 0.3,and reaction time 15 minutes.

EXAMPLE B5

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 2.2 A/dm², frequency 10 Hz, duty cycle 0.3, andreaction time 15 minutes.

EXAMPLE B6

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 1.6 A/dm², frequency 1000 Hz, duty cycle 0.6,and reaction time 15 minutes.

EXAMPLE B7

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 1.0 A/dm², frequency 1000 Hz, duty cycle 0.3,and reaction time 15 minutes.

EXAMPLE B8

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 1.6 A/dm², frequency 1000 Hz, duty cycle 0.3,and reaction time 10 minutes.

EXAMPLE B9

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, sodium metasilicate. (64 g/L) was used as a film thickeningagent, and trisodium phosphate (19 g/L) and potassium citrate (80 g/L)were used as a film adjustment agent. This example used a pulserectifier and adopted the following conditions: temperature 15° C.,electric current density 1.6 A/dm², frequency 100 Hz, duty cycle 0.3,and reaction time 15 minutes.

Control A:

Surface treatment composition: sodium hydroxide (70 g/L) was used as ahydroxide, and trisodium phosphate (50 g/L) was used as a filmadjustment agent. This example used a d.c. rectifier and adopted thefollowing conditions: temperature 20° C., electric current density 1.6A/dm², and reaction time 30 minutes.

Control B:

Surface treatment composition: sodium hydroxide (20 g/L) was used as ahydroxide, and sodium metasilicate (80 g/L), sodium carbonate (53 g/L),and boric acid (12.5 g/L) were used as a film thickening agent. Thisexample used a d.c. rectifier and adopted the following conditions:temperature 20° C., electric current density 1.6 A/dm², and reactiontime 30 minutes.

Table 1 lists the test results of corrosion resistance for the filmsprepared in the examples and controls, wherein the salt spray test forcorrosion resistance used 5% NaCl aqueous solution. A film is rated“Pass” if no corrosion spot is formed at 35° C. after 100 hours in thetest. Generally speaking, the thickness of the formed film is notrelated to the corrosion resistance of the film per se. The structureand the denseness of the formed film per se are important factorsaffecting the corrosion resistance of the film. Thus, the test resultsof the corrosion resistance (the salt spray test) for the formed filmsin the examples according to the present invention are shown togetherwith the impedance values in Table 1. When the impedance value is high,i.e. a higher denseness, the formed film will also have a bettercorrosion resistance.

The corrosion resistance of the film formed in Example A1 is the best inthe A-series examples, but is generally lower than that of the filmformed in the B-series examples. Therefore, a pulse rectifier seems tobe a better choice for the method of the present invention. However, thefilms formed by using a d.c. rectifier in the method of the presentinvention still have good corrosion resistance, referring to the testresults of the A-series examples in Table 1. TABLE 1 Test results forcorrosion resistance Film thickness, Impedance, Result of Roughness,Example μm Ω brine spray test μm A1 10 900K Pass A2 9.5 880K Pass A310.3 400K A4 5 350K A5 1.5 180K A6 9.7 500K A7 10 800K Pass B1 7.23000K  Pass 0.59 B2 13.6 2400K  Pass B3 5.3 700K B4 12.5 2600K  Pass 0.9B5 8.5 550K 0.7 B6 8.7 1400K  Pass B7 4.8 1200K  Pass 0.51 B8 5.5 1300K Pass 0.47 B9 12.8 1100K  Pass 1.37 A No film Not Pass deposited B 10-60150K Not Pass

The above-mentioned examples are for illustrative purpose only and notfor limiting the scope of the present invention, which is defined in theclaim appended.

1. A surface treatment method, which comprises: (A) providing a metal ofMg or Mg Alloy, a surface treatment composition, and a tank, whereinsaid surface treatment composition comprises a hydroxide, a filmthickening agent, and a film adjustment agent, wherein said tank isprovided with an electrode and said surface treatment compositiontherein; (B) immersing said Mg or Mg Alloy or a surface thereof intosaid surface treatment composition in said tank; (C) flowing an electriccurrent through said Mg or Mg Alloy and said electrode in said tank; and(D) terminating the electric current, and removing the treated Mg or MgAlloy from said tank.
 2. The method as claimed in claim 1, wherein theMg or Mg Alloy during said Step (C) for is at 0˜40° C.
 3. The method asclaimed in claim 1, wherein said surface treatment composition in Step(B) has a pH value greater than
 9. 4. The method as claimed in claim 1,wherein said film thickening agent in Step (A) is selected from thegroup consisting of aluminate, silicate, vanadate, molybdates,tungstates, and a combination thereof.
 5. The method as claimed in claim1, wherein said film adjustment agent in Step (A) is selected from thegroup consisting of potasium dihydrogen phosphate, sodium dihydrogenphosphate, tripotassium phosphate, trisodium phosphate, oxalic acid,succinic acid, fatty acid, malic acid, and a combination thereof.
 6. Themethod as claimed in claim 1, wherein said hydroxide in Step (A) isselected from the group consisting of sodium hydroxide, potassiumhydroxide, and a combination thereof.
 7. The method as claimed in claim1, wherein said surface treatment composition in Step (A) has aconcentration of said hydroxide of 10˜100 g/L.
 8. The method as claimedin claim 1, wherein said Mg and Mg alloy in Step (A) is a Mg-richedalloy or casting Mg alloy.
 9. The method as claimed in claim 1, whereinsaid surface treatment composition in Step (A) has a concentration ofsaid film thickening agent of 10˜150 g/L.
 10. The method as claimed inclaim 1, wherein said surface treatment composition s in Step (A) has aconcentration of aid film adjustment agent of 10˜300 g/L.
 11. The methodas claimed in claim 1, wherein said electric current in said Step (C) isflowed at an average electric current density of 1˜10 A/dm².
 12. Themethod as claimed in claim 1, wherein said Step (C) is carried out insaid surface treatment composition for 5˜240 minutes.
 13. The method asclaimed in claim 1, wherein the Mg-containing material is connected to apositive electrode of a rectifier in Step (C).
 14. The method as claimedin claim 13, wherein said rectifier is selected from the groupconsisting of a d.c. rectifier and a pulse rectifier.
 15. The method asclaimed in claim 14, wherein said d.c. rectifier is selected from thegroup consisting of a constant current type d.c. rectifier, a constantvoltage type d.c. rectifier, and a constant current density recycle typed.c. rectifier.